"Don't look a gift horse in the mouth"
You said what now?
"No man ought to looke a geuen hors in the mouth."
"Don't be an ungrateful jerk when you get a gift. Dick."
That's useful advice. But slow down, what the heck is a gift horse and why am I looking in its mouth?
A "gift horse" for those on the slow side, is a
horse that is given to you as a gift.
As horses age there teeth move slightly further and
so you can estimate a horses age by looking in their mouth
[see our feature on the phrase - "long in the tooth"].
Basically this is saying "Don't look under that Holden's bonnet if
someone is nice enough to give it to you for free".
The phrase first appeared in print in 1546.