어서 와.

The word '어서' (eoseo) means 'quickly' or 'at once,' and '오다' (oda) means 'to come.' '와' (wa) is the informal imperative form of '오다.' While the literal components suggest speed, its common usage has evolved to a general welcoming expression. It doesn't have a deep historical or mythological origin beyond the simple combination of these two common Korean words. The usage as a greeting has been prevalent for centuries in various forms, adapting with the evolution of the Korean language. It's a pragmatic and direct way to invite someone in or acknowledge their arrival, reflecting a cultural emphasis on hospitality and direct communication in informal settings.