1. In or to a place or situation in back of or to the rear of — used as a function word to indicate something that screens an observer; following in order.
2 — used as a function word to indicate backwardness, delay, or deficiency.
3. In the background of; out of the mind or consideration of; beyond in depth or time.
4. In support of; on the side of; with the support of.

1. In or into an inferior position.
2. In or to or toward the rear.
3. Remaining in a place or condition that has been left or departed from.
4. Of timepieces; SYN. slow.
5. In debt; SYN. behindhand, in arrears.

Buttocks — often used as a euphemism for ass in idiomatic expressions

Having the lower score or lagging position in a contest; SYN. trailing.