'jar-shaped' is a compound formed from 'jar' and the past participle 'shaped' (from the verb 'shape'). 'jar' originates from Old French, specifically the word 'jarre', where it meant 'earthenware pot'. 'shape' originates from Old English, specifically the verb 'sceapian'/'sceapan', where it meant 'to form or create'.

'jar' entered Middle English from Old French 'jarre'; 'shape' developed from Old English roots and formed the noun/verb 'shape' in Middle English. The compound 'jar-shaped' arose in Modern English by combining the noun 'jar' with the past participle 'shaped' to describe something having the form of a jar.

Initially the elements referred simply to the container 'jar' and the action/idea of 'shaping'; over time they combined into an adjectival phrase whose meaning is now 'having the shape of a jar'.