"Fast" in that sense is to be firm or close, and not really a measure of speed. Held Fast = held closely, firmly or tightly. Also, "fast' (as in Held Fast) I believe would be an adverb, as it describes the verb 'to hold'
Good marks, but I'm afraid they aren't quite the opposites.
Actually, the word I was looking for is
Cleave.
- to divide by a blow; to split; to sever
- to stick to; to adhere; to be united closely; to remain faithful (to)
So, Cleave (to split apart) and Cleave (to adhere closely to), identical by name, opposites by nature.