1. What does it mean?
칼답 describes answering with little or no apparent delay. You can praise someone's quick response or say 칼답하다 about replying as soon as a message arrives.
Also written or said as 칼답하다, kaldap
2. The nuance
There is no fixed number of seconds that qualifies. A fast reply does not prove romantic interest, and a slow one does not prove dislike. In a busy conversation, it may simply mean the person already has their phone open.
3. Examples
“파일 위치 물어보자마자 칼답해 줘서 금방 찾았어.”
You replied immediately when I asked where the file was, so I found it quickly.
“수업 중에는 칼답 못 해. 끝나고 연락할게.”
I can't reply instantly during class. I'll message you afterward.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.