واس

Etymology
A preposition not attested in Classical Persian and not heard frequently in any area outside of.

It’s from a language. Compare 🇨🇬, and 🇨🇬.

This would make sense because the people of Tehran and much of the surrounding area were speakers before being Persianized, which happened after  became the capital of the.

Compare also 🇨🇬, and 🇨🇬. The form is also heard in languages lurish, Southwestern Fars, Lari(vasoy وَسُی) and many southern dialects.

Preposition

 * 1)  for,  by reason of, for the sake of