also: よう · よお
interjection
Casual, friendly call to get someone's attention or greet them. Often used among friends or in informal situations.
よー、元気か？
Hey, how are you?
よー、久しぶりだな！
Yo, long time no see!
particle
Empathetic, urging particle; a lengthened, more emotional version of よ. Used to strongly encourage, warn, or draw attention in a friendly or familiar way.
See also: よ
早くしろよー。
Come on, hurry up.
大丈夫だよー、心配するな。
It's okay, hey, don't worry.
Lengthened form of the interjection/particle よ, used for added emphasis or friendliness. The exact origin of よ itself is uncertain, but it has been used as a sentence-final particle since at least the Edo period.