骨の折れる (ほねのおれる, hone no oreru) is a common idiomatic expression that means “very tiring/tough”. Its literal meaning is “bone breaks”, as in “骨の折れる音 (bone-breaking sound)”. However, it can be also used as an idiom to describe something tiring or tough, in which case it modifies a noun like an adjective. For example, “骨の折れる仕事” means “very tiring work” and “骨の折れる作業” means “very tiring task”. When you want to say “something is tiring/tough” using this expression, you can replace “の” with “が” in the phrase and use it like “この仕事はとても骨が折れる” meaning “This work is very tiring”. However, since “骨が折れる” also means ‘(one’s) bones break’, it is clearer to add some adverb like “なかなか (fairly, really, quite)” or “とても/かなり (very)” before this expression to indicate that you’re using it as an adjective.
Related Expression “骨折り損のくたびれ儲け”
“骨折り損のくたびれ儲け” is a famous proverb that means “After you’ve made a lot of effort, you’ve got nothing but tiredness in the end”. In this saying, “骨折り” describes your big effort, “損” means “loss”, and “くだびれ儲け” literally means “earn tiredness”, an ironical phrase meaning that your effort hasn’t paid off in the least.
Translation is 骨の折れる (very tough) work.
Synonyms: 大変 (tough), 疲れる (tiring)
Synonyms in English: laborious, tough, tiring, painstaking, backbreaking
Related words and phrases: 骨 (bone), 折れる (crack, bend), 骨折り損のくたびれ儲け