A closely fought contest where the outcome is in doubt is said to be “nip and tuck.”
It equates to the expression “blow for blow,” when the advantage keeps changing from one competitor to another.
The answer is in the original aggressive meanings of the two words.
A nip was, and still is, a bite, while a tuck was a small, narrow dagger used by artillerymen when overrun and forced into hand-to-hand combat.
“Nip and tuck” literally means a vicious, life and death struggle.
