Arse/ɑːrs/ Definition 1 of 3Pro Tip 1/3Informal Slang Arse is very informal and often considered rude; use in casual settings only. Oops, I've just texted my boss 'Arse over tit', that was inappropriate. Pro Tip 2/3Not for Formality Avoid 'arse' in formal writing or speaking, instead use 'buttocks' or 'rear end'. In the medical report, he wrote 'buttocks' instead of 'arse'. Pro Tip 3/3Humorous Insult Used jokingly among friends, but can offend if the relationship isn't close. My mates laughed when I said, 'I fell on my arse'.