noun which may take the genitive case particle 'no'
Slang reduplication of 混む, emphasizing a very crowded state. Used casually, often in spoken language or informal writing.
週末のショッピングモールは混み混みだった。
The shopping mall was super crowded on the weekend.
電車が混み混みで、身動きが取れなかった。
The train was so packed I couldn't move.
Reduplication of the verb stem 混み (from 混む, 'to be crowded') for emphasis, common in colloquial Japanese.