also: いっぴつかく
godan verb (-ku), intransitive verb
write a few lines; drop a line; write a note
Used when writing a short informal message or letter to someone. Often implies a brief, casual communication.
近況を一筆書いて送った。
I dropped a line to let them know how I've been.
お礼の一筆を書くのを忘れていた
I forgot to write a short thank-you note.