びっしり

〈場ば所しょ〉に〈物もの〉がびっしり詰つまるfor things to be packed tightly into a place
本ほん棚だなに本ほんがびっしり詰つまっている。
The bookshelf is packed tightly with books.
びっしり〈動どう詞し〉to do something thoroughly and without a break
毎まい日にちびっしり練れん習しゅうする。
I practice thoroughly and without a break every day.
予よ定ていがびっしりだto have a schedule packed full of things to do
今こん週しゅうは予よ定ていがびっしりで、休やすむ時じ間かんがない。
My schedule this week is packed full, so I have no time to rest.
〈物もの〉にびっしり字じを書かくto write densely, filling a surface with tightly packed writing
ノートにびっしり字じを書かいた。
I filled the notebook with densely packed writing.

Related words

ぎっしりN3sense 1
ぎっしり emphasizes being crammed to full capacity, while びっしり focuses on things being closely packed with no gaps, often in rows or lines.
みっしり sense 2
みっしり is used for intense, thorough effort like training or studying, while びっしり in this sense means doing something continuously without a break.

まばらN1sense 1
まばら describes things scattered thinly with gaps, the opposite of びっしり's densely packed, no-gap feel.
スカスカN1
びっしり describes something packed tightly full, the opposite of the hollow, gap-filled sense of スカスカ.

ぎっしりN3
ぎっしり and びっしり sound alike, but ぎっしり highlights things packed tightly into a space (a bag, a schedule), while びっしり highlights a surface densely covered without gaps (rows of teeth, sweat, hair).

Onomatopoeic/mimetic word; the exact derivation is uncertain, but it likely imitates the sound or feeling of things being packed tightly together.