Casual greeting used among friends or in informal situations. Similar to 'hey' or 'hi' in English.
やあ、久しぶりだね。
Hey, long time no see.
Expression of mild surprise, realization, or admiration. Can be used when noticing something unexpected.
やあ、これはすごいね。
Wow, this is amazing.
yah!; heave-ho!
Shout used when exerting effort, such as lifting something heavy or making a vigorous move. Often part of a repeated chant like えいや.
See also: えいや
やあ、やあ、と声をかけて重い箱を持ち上げた。
With a 'heave-ho, heave-ho,' they lifted the heavy box.
やあ is a simple casual greeting like 'hi', while よー can be used the same way but also doubles as an emphatic urging particle.
やあ is a native casual greeting between friends, whereas ハロー carries a foreign, English-flavored casualness.
こんにちは is the standard polite greeting used in most situations, while やあ is a casual, informal way to say hi among friends.
The origin is uncertain; it is a natural exclamation found in many languages, possibly onomatopoeic or a spontaneous vocalization.