also: きゅうかつをじょする
expression
apologize for long neglect of friends
Archaic set phrase used in letters or formal speech to express regret for not having been in touch for a long time.
See also: 久闊を叙する
手紙の冒頭で「久濶を叙す」と書くのは、昔の書簡の決まり文句だった。
Writing '久濶を叙す' at the beginning of a letter was a set phrase in old-fashioned correspondence.
ご無沙汰 is the modern, everyday expression for 'long silence' or 'neglecting to keep in touch', while 久濶を叙す is an archaic, formal set phrase.
The phrase is composed of 久濶 (long interval/neglect) and 叙す (to express/state). The exact historical derivation is uncertain, but it follows classical Chinese-influenced letter-writing conventions.