Literal meaning: a single item or object. Often used in set phrases like 一物もない (not a single thing).
There was not a single thing on the desk.
ulterior motive; secret intention; plot
Implies a hidden agenda or scheme behind someone's actions. Often used in the phrase 一物がある (to have an ulterior motive).
There must be an ulterior motive behind his sudden kindness.
That proposal seems to have a hidden agenda.
Euphemistic slang for the male genitalia. Usually written in katakana as イチモツ. Not used in formal contexts.
He always talks about his junk.
Rare euphemism for money, possibly from the idea of 'one valuable thing'. Not common in modern speech.
In old novels, 一物 is sometimes used as a euphemism for money.