x1 is a polygon with x2 sides.
sor-jga-tai < so'i + jganu + tarmi. A calque of "polygon" (poly- many, -gon angle). [Even though "many" here actually means "3 or more", so'i was chosen for a shorter resulting compound.] Arbitrary polygon names are formed with rafsi numbers plus -jgatai: cibjgatai, vonjgatai, mumjgatai, etc.