Even though the proposed connection between חמש and the meaning ‘five’, then, the derivative expression ‘fifth (rib?)’ is intriguing, it seems to me it is untenable.
Some factors we must consider:
The term is used only 4 times in the TaNaKh (2 Sam 2:23; 3:27; 4:6; 20:10).
Anyway – let’s get things straight immediately: we have to remember that the Bible doesn’t mention any ‘rib’ in these passages. This kind of translation is based not only on the assumption that the Hebrew verb at issue is linked with the concept of ‘five’, but – also – that this ‘5-concept’ is linked, in turn, with the numbering of ribs, the body part more aimed from the cold steels.
As regards one of these passages - 2 Sam 4:6 (the murder of Ish-Bosheth) - did could be possible, from Rechab & Baanah’s part, to aim exactly to the Ish-Bosheth’s fifth rib.
As regards another of the passages - 2 Sam 20:10 (Joab kills Amasa) we have to see the matter otherwise, on the basis of the context. In fact, this passage specifies that the Joab’s sword was in his left hand, as he took hold Amasa’s beard with his right hand. Tough I’m not a swordsman, it seems to me it’s very improbable such a body sequence.
The similar passage of 2 Sam 4:6 (Joab kills Abner) doesn’t specify the hand used by Joab to strike Abner, but since Joab was left-handed – as we have seen in 20:10 – this omission is ininfluent. If Joab were ambidextrous the specification of the hand utilized for the murder (as in 2 Sam 20:10) would be useless. So, the same remarks we’ve made for 20:10 are sound also for 4:6.
The last passage (2 Sam 2:23) fully confirms - again - this conclusion (i.e. no connection with a ‘five’ concepts). It speaks about the killing of Asahel. From the Bible account we can imagine Abner (identifying ourselves with him) - that was running ahead Asahel – smiting him (Asahel) with the butt end of his (Abner’s) spear, while he (Abner) was seeing from the corner of his eyes, obviously. Unless Abner was a professional contortionist (in addition to his warrior expertise), a so precise (“calculated”, as ‘Michael L Greene’ said) stroke was impossible (only casual, at best).
See, please, how Robert Alter – in his translation – explains this body sequence:
“Asahel is pursuing Abner at top speed. Abner, to save his own life, uses an old soldier’s trick: he suddenly stops short and thrusts his spear backward, under his pursuer’s shield (if Asahel is carrying one) and into the soft belly. The momentum of Asahel’s rapid running would have contributed to the penetrating force of the spear’s butt.”
Consistently, Alter translates this stichus: “...struck him in the belly”.
So, the higher probability is that the term חמש possesses no connection with the sense of ‘five’, but with the sense of ‘to fatten’, so the nouns ‘abdomen’, ‘stomach’, and so on.
This verb (חמש) is present in MT in a number of graphical variants. I mention here only one of them, to simplify the matter, חשם, that is different from the verb we are speaking only for the permutation of the last two letters (שם instead of מש). All the graphical variants of חמש possess the same meaning.
Not everybody knows that there are a lot of MT Hebrew verbs that possess a number of graphical variants, risen from some consonantal permutations (a kind of anagrams). I present here only three examples, but the phenomenon happens fairly often.
For example, the concept of “to be foolish” is present in כסל (KSL), as in Jer 10:8, but – also – in סכל (SKL), as in Gen 31:28. Note the permutation KS > SK.
The concept of “to mould” is present in ארץ (ARJ), as in Gen 1:1 (derivate noun, ‘earth’), but – also – in יצר (IJR), as in Gen 2:7. Note the permutation RJ > JR.
The concept of “to be powerful” is present in אמץ (AMJ), as in Gen 25:23, but – also – in עצם (OJM), as in Gen 2:7. Note the permutation MJ > JM.
As a consequence of this fact, there are a number of scholars who sustain the link between the sense of ‘to fatten’ (and not with that of ‘five’), and then, they produce a ‘belly’-alike translation. We may cite here (bold is mine):
Johann Peter Lange (his Commentary, on 2 Sam 3:27): “2Sa 2:23. חמֶשׁ. Not one of the ancient VSS. renders this word ‘fifth rib’, Sept[uagint], ‘loins’ (ψόα), Syr. ‘breast’, Chald. ‘side of the loins’, Vulg ‘inguen’ […] Gesenius and Fürst connect the word with a root (found in Arabic), meaning ‘to be fat or strong’.” Note how ancient codex and versions confirm the ‘belly’-alike translation.
Albert Barnes: “The word so rendered here (and in marginal references) means the abdomen, and is not etymologically connected with the Hebrew for ‘five’, as the translation ‘fifth rib’ supposes, but with a verb meaning ‘to be fat’, or ‘strong’.”
Cambridge Bible: “The E. V. follows the Jewish commentators in thus rendering a word which occurs in three other passages of this book (2Sa 3:27, 2Sa 4:6, 2Sa 20:10) and nowhere else. ‘In the belly’ is however the more probable meaning.”
Adam Clarke (on 2 Sam 20:10): “In the fifth rib - I believe חמש chomesh, which we render here and elsewhere the fifth rib, means any part of the abdominal region. The Septuagint translate it ‘την ψοαν’, ‘the groin’; the Targum, ‘the right side of the thigh’, i.e., (the phrase of the Targumist being interpreted), ‘the privy parts’. That it means some part of the abdominal region, is evident from what follows, ‘And shed out his bowels to the ground. It appears from this that, in plain English, he ripped up his belly’.
In addition, I present here a brief list of various translations which follow the belly-groin-stomach track (on the basis of 2 Sam 2:23, as a sample passage):
“abdomen”, Amplified Bible, ISV, Modern English Version, NAB, The Voice;
“belly”, Alter, Darby, LITV, MKJV, Names of God Bible, NJB;
“body”, ASV, GNT, Hebrew Names Version, Holman, World English Bible;
“groin”, Douay-Rheims, JPS, TLV;
“stomach”, BBE, CEV, ESV, Evangelical Heritage Version, Expanded Bible, GNB, HCSB, LEB, Lexham, New Century Version, NIRV, NIV, TS2009;
“ventre” [French for ‘belly’], Luis Segond;
“Bauch” [German for ‘belly’], Luther.
@Emmanuel Adediwura: I hope these information will be useful for your research.