especially; particularly; wholeheartedly
Used to emphasize that something is done with special focus or intensity, often in the phrase 立てて～する (to do something especially/particularly). The 'wholeheartedly' nuance appears when the action is done with strong intention.
He said there is no particular reason to oppose it.
Since I didn't have anything in particular to do, I went home early.
particularly; especially; in particular
Derived from the verb 立てる (to stand, to set up) in its continuative form, used adverbially. The exact semantic shift to 'especially' is uncertain, but it may relate to the idea of 'setting something apart' or 'establishing a focus'.