particle
sentence-final particle for emphasis
Familiar, masculine-leaning particle that strengthens a question, assertion, or invitation. Similar to かい, だい, and わい, but with a blunt, emphatic feel. Often used in casual male speech.
何してるんだい？
What are you doing?
そんなことあるかい。
Is that really the case?
Similar emphatic particle, but often associated with older or dialectal male speech; い is more common in modern casual usage.
The exact derivation is uncertain; it may be a contraction of かい or a related emphatic particle. It is used in familiar, often masculine, speech.