A submarine.
A submarine sandwich: a sandwich made on a long bun.
We can get subs at that deli.
A substitute, often in sports.
She worked as a sub until she got her teaching certificate.
(often in plural) A subscription: a payment made for membership of a club, etc.
A subtitle.
I've just noticed a mistake in the subs for this film.
A subroutine (sometimes one that does not return a value, as distinguished from a function, which does).
A subeditor.
A subcontractor.
(BDSM) A submissive.
A subordinate.
A subaltern.
A subscription (or (by extension) a subscriber) to an online channel or feed.
I'm totally stoked; just got 10 new subs after my last video.
Subsistence money: part of a worker's wages paid before the work is finished.
To substitute for.
To work as a substitute teacher, especially in primary and secondary education.
To replace (a player) with a substitute.
He never really made a contribution to the match, so it was no surprise when he was subbed at half time.
Less commonly, and often as sub on, to bring on (a player) as a substitute.
He was subbed on half way through the second half, and scored within minutes.
To perform the work of a subeditor or copy editor; to subedit.
To lend.
To subscribe.
(BDSM) To take a submissive role.
To coat with a layer of adhering material; to planarize by means of such a coating.
To prepare (a slide) with a layer of transparent substance to support and/or fix the sample.
Under.