In 1 Cor. 9:27, Paul says:
But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified. (ESV)
What is meant by being "disqualified"?
This is a fascinating passage. In 1 Corinthians 9:24-27, Paul is using an athletic metaphor as both an example of the Christian life and a particular defense of his ministry. Within that context, we could consider this a reference either to losing (not winning) - verse 24, failing to compete effectively - verse 26, or to being rejected from competition. (Perhaps for breaking the rules?)
The Greek word translated "disqualified" is αδοκιμος (Strongs G0096), meaning "failing the test", "rejected" or "worthless". This word appears 8 times in the New Testament. In addition to "disqualified", the NET Bible also translates it "unfit", "useless" or "fail". This is also the word translated "depraved" or "debased" in Romans 1:28.
(My personal opinion is that it's one of the first two that I mentioned above, since there are already verses in the passage that refer to them. Also, the two concepts are very closely related - if you don't compete effectively, you can't win.)
See Hebrews 12:1-2 for another passage with a very similar metaphor.
St. Paul is speaking of whether or not he will be a partaker in the gospel (salvation, life in Christ). He compares the people of God in the Old Testament not partaking of what it was they had progressed toward and been enjoying. They had even 'drank of Christ.' They were 'baptized.' Yet they fell into damnable sin and did not repent (Rom 8:13; 11:22; Rev 2:5; 2:16; 2:21 etc.).
1 Corinthians 9:23-10:121
Now everything I do, I do for the gospel, to the end that I might become a partaker in it myself. Do you not know that those who run in the stadium all run but only one recieves the prize? Race you in like manner, that you might take hold of the prize. Now everyone that contends in the race disciplines himself, that he might obtain what is but a corruptable crown: but we, an incorruptable. So then, I race, not irresolutely, nor do I box as one punching air, but instead I punish my body, and subdue it, lest it happen that after preaching to others, I myself should be disqualified.2
For I do not want you to be ignorant, brothers, that our forefathers were all under the one cloud, and all passed through the sea, and all were baptized into Moses in the cloud and the sea, and all ate the same spiritual food and drank the same spiritual drink. For they drank of the same spiritual rock with followed them: and that rock was Christ. Nevertheless, the majority of them did not please God, for they died, and were left strewn across the wilderness. Now these things would utlimately typify things pertaining to us: namely, to teach us not to be ones chasing after evil as they chasted after it, and not to become idolators as some of them, as also it is written: The people sat down to eat and drink, and and rose up to play. Moreover, that we might not commit sexual sins, as some of them did and fell every one in a single day, twenty-three thousand; nor tempt the Lord as some of them tempted and perished by the serpents; nor complain as some of them complained and perished by the destroyer. Yet all these things befell them as an example: they were written for our admonition upon whom the end of ages has come. So that he who thinks himself to stand to should beware, lest he fall.
The reason St. Paul is 'preaching to others' the gospel is to save their souls (1 Cor 9:22). So when he says 'that after preaching to others' (what) 'I myself might become a partaker in,' he clearly means 'the gospel,' which he is preaching; and he expresses that he might not become a partaker in its promises (like the examples he provides of the Old Testament people of God): 'lest it happen that .. I myself should be counted unfit.' He predicates this upon discipline of the body (Rom 11:22; Col 1:24). St. Paul uses a strong word (γαρ) meaning 'because I will prove what I just said' (here and almost always translated 'for') to link disqualification in the race with the destruction of those people of God who apostatized and died in sin.
See also Revelation 2:10.
1 I provided a fresh translation to eliminate unnecessary inconsistencies which obscure the contextual issues. Italics are to hepl understand what is meant; bold is used for highlighting the significant portions relevant to the question, and my answer.
2 St. Paul, significantly, uses this 'disqualified' elsewhere, such as in Romans 1:28, of which the meaning is 'reprobate; debased:' "And just as they did not see fit to acknowledge God, God gave them up to a reprobate [αδοκιμον] mind to do the unseemly." Even though the context shows that St. Paul is using it in the sense that he is judged not to have succeded in running the race, this overtone may not be overlooked—especially, as I point out in my answer, the race is living according to Christ so that you don't forfeit salvation by unrepentant sin. cf. Phil 2:12.