יִשָּׂשכָ֖ר חֲמֹ֣ר גָּ֑רֶם רֹבֵ֖ץ בֵּ֥ין הַֽמִּשְׁפְּתָֽיִם׃
יִשׂשָׂכָר איז אַ בײניקער אײזל, ער הױערט צװישן די שאָפֿגעהעפֿטן.
Rashi
יששכר חמר גרם means ISSACHAR IS A BONY ASS — he bears the yoke of the Torah (i.e. obligations) like a strong ass upon which may be placed a heavy load (Genesis Rabbah 99:9).
Chizkuni
רובץ בין המשפתים, “crouching between boundaries.” His territory was hemmed in on both sides between his tribal brothers as spelled out in Joshua 19,10-23.
Da'at Zekenim
יששכר חמור גרם, “Issachar is a bony donkey;” in the Talmud tractate Niddah, folio 31 this line is understood as Yaakov saying that a bony donkey was the cause of Issachar having been born. How so? When Yaakov came home from the field and Leah heard the donkey he was riding on braying, she went out of her tent to meet him and brought him to her tent. During that night she became pregnant with Issachar. (B’reshit Rabbah 99,10)