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