骨の折れる (ほねのおれる, honenooreru) is a very common expression that means “very tiring/tough”. Its literal meaning is “(my) bone cracks” and can be used as it is, like “骨の折れる音 (sound of a bone cracking)”. However, usually it is used as an idiom to describe something very tiring or tough, and modifies a noun like an adjective. For example, “骨の折れる仕事” means “very tough 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, replace the “の” with “が” in the phrase, and use it like “この仕事はかなり骨が折れる” meaning “This work is very tiring”. However, note that “骨が折れる” is also often used when you actually break your bones, so 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.
Incidentally, “骨折り損のくたびれ儲け” is a famous proverb that means “You’ve made a lot of effort but in you have 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 irony indicating that your effort hasn’t paid off in the least.
Translation is 骨の折れる (very tough) work.
I finished my assignment within a day. It was really 骨が折れた (very tough).
(note) “骨が折れた” is the past form of 骨が折れる
Synonyms: 大変 (tough), 疲れる (tiring)
Synonyms in English: labourious, 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