true intention; underlying motive
Refers to a person's real, often hidden, intention or motive behind their words or actions. Commonly used in expressions like 底意を見抜く (see through someone's true intentions).
I felt there was an ulterior motive behind his kindness.
It's difficult to see through someone's true intentions.
Compound of 底 (そこ, 'bottom') and 意 (い, 'mind, intention'), literally 'bottom intention', referring to the true intention lying beneath the surface.