The process of apposing; interrogation, examination; a posing question, a puzzle.

A proposition that ‘gravels’ one, a ‘poser’.

A difficult or perplexing question; a puzzle. Also: a tricky or intractable problem.

That which (or one who) wrings or causes contortion; esp. figurative something that confounds, non-plusses, or ‘doubles up’, a ‘staggerer’ (slang)…

A matter which nonplusses a person.

figurative. A state or condition of obscurity or confusion; something designed to confuse or obscure.

Something that perplexes or overwhelms the mind.

Capable of being, or liable to be, confused. Also as n. plural (and with spelling -ible), things, esp. words, that may be confused.