Spanish preposition 'so'

Question

This is a question about the Spanish preposition 'so' ( not the English adverb or conjunction 'so' )

Volvieron a su país so pena de ser arrestado

which Google translates as

They returned to their country or risk being arrested

I have been learning Spanish for quite a long time and only to-day have I come across this Spanish 'so'.

I mean it's not uncommon for a learner of a non-native language not to come across some nouns, adjectives, idioms etc., until they have studied the language for a lot of years, but prepositions?

I am curious as to how many other students had also never heard of this preposition until now?

Answer 1

It's use is mainly formal and a better translation would probably be "under the pain of arrest", which carries the same degree of quaint formality.

Answer 2

There's a reason for that. "So" as a preposition has been replaced almost completely by "bajo" or "debajo"; it only lingers in expressions like the one you mentioned ("so pena de"). As an adverb you can see it in expressions like "¡so listo!".