On the pretext of: 〜にかこつけて

〜にかこつけて means “on the pretext of”: you hide behind a perfectly real reason (a business trip, an illness, the rain) to actually do something else that suits you. The expression almost always carries a reproach or a touch of irony. Written, critical register.

• 部長は出張にかこつけて、京都の紅葉を見物してきたらしい。ぶちょうはしゅっちょうにかこつけて、きょうとのこうようをけんぶつしてきたらしい。Apparently the manager used his business trip as cover to go and admire the autumn leaves in Kyoto.
• 父は散歩にかこつけて、毎晩近所の居酒屋に寄っている。ちちはさんぽにかこつけて、まいばんきんじょのいざかやによっている。On the pretext of going for a walk, my father drops into the local pub every evening.
• 体調不良にかこつけて、気の進まない会合を欠席した。たいちょうふりょうにかこつけて、きのすすまないかいごうをけっせきした。I used feeling unwell as an excuse to skip a meeting I didn't fancy.

The verb かこつける (託ける in kanji, but almost always written in kana) means “to invoke something as a pretext”. So 〜にかこつけて means “using A as cover” to do B.

The key point: A really exists. There really is a business trip, a walk, rain. But that real situation is hijacked to do something else, which you wouldn't have dared do openly — like English “under the guise of”, “using X as an excuse”.

• 彼は取材にかこつけて、有名店をはしごしている。かれはしゅざいにかこつけて、ゆうめいてんをはしごしている。Under the guise of research for an article, he goes from one famous restaurant to the next.
• 法事にかこつけて、親戚が集まって酒盛りをした。ほうじにかこつけて、しんせきがあつまってさかもりをした。On the pretext of a Buddhist memorial service for a deceased relative, the family got together for a drinking party.

The expression attaches to a noun. For a clause, you nominalise it with こと. Before a noun, you use the た-form: 〜にかこつけた + noun.

• 忙しいことにかこつけて、部屋の掃除を何か月もしていない。いそがしいことにかこつけて、へやのそうじをなんかげつもしていない。Using being swamped as my excuse, I haven't cleaned my room for months.
• 視察にかこつけた海外旅行が問題になった。しさつにかこつけたかいがいりょこうがもんだいになった。A trip abroad disguised as an inspection tour caused a scandal.

〜にかこつけて isn't neutral: the speaker is judging the hijacked action, either in others (criticism, press irony) or in themselves (self-mockery). You often find it in articles about public spending or in slightly shamefaced confessions.

If you simply want to give a reason without judging, this isn't the right expression.

• 議員が視察にかこつけて観光していたことが報じられた。ぎいんがしさつにかこつけてかんこうしていたことがほうじられた。The press revealed that lawmakers had been sightseeing under cover of an inspection tour.

All four expressions are about a reason put forward, but not with the same intention. 〜を口実に is more direct and common: the pretext may even be made up. 〜をいいことに is “to take advantage of a favourable situation” (often a lack of supervision) to misbehave. 〜を理由に stays neutral: it's the official reason.

• 先生がいないのをいいことに、生徒たちは騒ぎ始めた。せんせいがいないのをいいことに、せいとたちはさわぎはじめた。Taking advantage of the teacher's absence, the pupils started messing about.
• 大臣は健康上の問題を理由に辞任した。だいじんはけんこうじょうのもんだいをりゆうにじにんした。The minister resigned on health grounds.