From the above link, we can learn that "lord" Strong's Hebrew 113, אָדוֹן, comes from an older, unused root which means "to rule".
From Gesenius's Lexicon at the same link, we can learn further that in addition to ruling is added the meanings of; to judge, to command, to lead, to control, and is a title of respect toward one who is noble and deserving of the respect, and is applied to princes, kings, rulers, and fathers.
To say that this word might mean Sarah was subject to the whims of her husband, would be to belittle the meaning of this term. Her use of the word in context implies respect toward her husband, and that she is subordinating herself respectively.
This is in keeping with what Eve was told would be the result of the fall, and how it would effect her, in her relationship toward her husband. (from the Genesis account 'your husband will rule over you, and your desire will be toward him'.)
Sarah's expression about Hagar and Ishmael being sent away, may have been extreme, and out of character for her. Hebrew scholars speculate from the text that describe's Ishmael's behaviour toward Issac, what it actually entailed, which Sarah observed, that upset her.
Some write there may have been a plot afoot by Hagar and her son, because of jealousy and competition; the text clearly bearing out that Sarah had problems with Hagar before Issac's birth, as well.
Abraham did not 'give in' to a whim of Sarah's, nor did he subordinate himself to her. He received divine direction from God about his household war, and he obeyed the Lord in how to resolve it. This resolution from God considered much more than restoring harmony to the home. According to some Hebrew scholars, it may have saved Issac's life.
It is interesting to note the choice of wording from the text, the Lord telling Abraham to go along with what Sarah requests. This does not indicate that Sarah usurped her husband's authority, nor does it indicate the Lord's approval of usurping.