Refers to the first harvest, catch, or product of a new season. Often carries a sense of freshness and special value.
I ate the first peaches of the season this year.
The first saury of the season have started appearing at the market.
Slang or euphemistic use for a person who has not had sexual experience. Less common than the seasonal meaning.
He seems to be still a virgin.
Compound of 初 (hatsu, 'first') and 物 (mono, 'thing'). The slang sense is a metaphorical extension from the idea of something being new or unused.