adverb
tightly packed; without gaps; all the time
Describes things being crammed together with no space, or an action happening very frequently or continuously. Often used with verbs like 詰める, 並べる, or 書く.
箱にリンゴがびっちり詰まっている。
The box is tightly packed with apples.
彼は毎日びっちり練習している。
He practices every single day without fail.
The word is likely a mimetic adverb derived from the sound-symbolic root びっ (suggesting tightness or closeness) plus the adverbial suffix り. The exact historical derivation is uncertain.