Where does “יאָסל” come from?

יאָסל (Yiddish) comes from Yiddish יוסף, from Hebrew יוֹסֵף — shall add.

Ancestry of “יאָסל”, step by step

Step	Language	Word	Meaning
1	Yiddish	יוסף	Joseph
2	Hebrew	יוֹסֵף	shall add