Green’s Dictionary of Slang

damned adj.

also damb’d, dam’d, demmed, domned

1. [late 16C+] a strong expression of reprehension or dislike; thus damnedest [the ‘Damned Crew’ (cite 1610) were a specific gang of violent aristocratic rowdies].

2. [18C+] a general intensifier, complete, utter.

In derivatives

damnedest (adj.)

a general intensifier, the most, the best; thus as n. in one’s damnedest, one’s utmost.