A lot vs. Alot: 9 Grammatical Pitfalls
1958, from over- + kill (v.). Originally in reference to nuclear arsenals; the general sense is from 1965. The verb is attested from 1946.
An excess, esp of needed action: Going twice would be overkill, don't you think?
[1958+; fr the use of the term in connection with the killing potential of nuclear arms and arsenals]