Meaning of 固約束 in Japanese

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Definition of 固約束

かたい(katai) 【硬い ·固い ·堅い ·緊い】

(adj-i) hard; solid; tough
stiff; tight; wooden; unpolished (e.g. writing)
このふたは固くて取れない。
This lid is too tight for me to open.
strong; firm (not viscous or easily moved)
safe; steady; honest; steadfast
obstinate; stubborn
bookish; formal; stuffy
固いこと言うなよ。
You don't have to be so formal.

やくそく(yakusoku) 【約束】

(n, vs) arrangement; promise; appointment; pact; engagement
ジムは映画に行く約束をしたガールフレンドに待ちぼうけをくわされ、雨の中を一時間も待たされて頭にきている。
Jim's angry because his date for the movie stood him up and he wasted an hour waiting for her in the rain.
convention; rule