What does "dirty snowball" mean?

A comet whose nucleus is composed of an amalgamation of rock, dust, water ice, and frozen gases.

What is a synonym for "dirty snowball"?

Common synonyms include: comet.

noun, slang

noun, slang ·Uncommon ·College level

Noun

1
A comet whose nucleus is composed of an amalgamation of rock, dust, water ice, and frozen gases. informal