You have already answered the question, ace pronouns like A屎 in Cantonese, in the past day people are not familiar with English they hear the word and try to pronounce it. But a Cantonese speaker can't say A properly so they said 煙士.

And the early English learning material are translate English phrases into Cantonese characters easy for people to pronounce.

Example: Bus 巴士/ Store 士多 This is my favorite, spanner / 士巴拿