also: ゆきがけのだちん
expression
while you're at it; taking the opportunity; on the way
Idiomatic expression meaning to do something incidentally or while already engaged in another activity, often implying making good use of the opportunity. Can be used literally for doing something en route, or figuratively for any convenient chance.
買い物に行くなら、行きがけの駄賃に郵便局にも寄ってきて。
If you're going shopping, stop by the post office while you're at it.
出張の行きがけの駄賃で、ついでに観光もしてきた。
I took the opportunity of the business trip to do some sightseeing on the side.