also: ところせしと
adverb
Describes a space so full of people or things that it feels cramped or overflowing. Often used with verbs like 並ぶ, 置く, or いる to emphasize abundance in a limited area.
会場には所狭しと人が並んでいた。
The venue was packed with people lined up.
机の上に所狭しと資料が置かれている。
Documents are placed all over the desk, leaving hardly any space.
Contracted reading variant; rarely used in modern writing.
From 所 (tokoro, 'place') + 狭し (semashi, classical adjective meaning 'narrow') + と (to, adverbial particle). The classical adjective form 狭し is the terminal form of 狭い in classical Japanese, used here adverbially. The contracted reading ところせしと drops the ま, reflecting a common historical sound change.