adverb
Indicates that something happened a very short time ago, often with a nuance of immediacy or 'just this moment'.
ついさっきまでここにいたのに。
He was here just a moment ago.
adverb
quite near; close by
Used for spatial or temporal proximity, often in the pattern ついそこ/そば/近く etc.
ついそこに郵便局があるよ。
The post office is just over there.
adverb
unintentionally; without thinking; by mistake
Expresses doing something impulsively or against one's better judgment, often with a sense of regret or helplessness.
ダイエット中なのに、ついケーキを食べてしまった。
Even though I'm on a diet, I ended up eating cake without thinking.
The origin is uncertain; it is a native Japanese adverb with no clear kanji etymology.