noun which may take the genitive case particle 'no', na-adjective
jam-packed; packed like sardines
Describes a space, container, or vehicle that is extremely crowded or tightly filled with people or things. Often used with に or の.
通勤電車はいつもぎゅう詰めだ。
The commuter train is always jam-packed.
かばんに荷物をぎゅう詰めにした。
I packed my bag tightly with stuff.
満員 means 'full house' or 'capacity crowd' and is more formal; ぎゅう詰め emphasizes the physical tightness and discomfort of being packed.
すし詰め is a near-synonym also meaning 'packed like sardines', but ぎゅう詰め is slightly more colloquial and onomatopoeic.
From the onomatopoeic adverb ぎゅうぎゅう (squeezing tightly) and 詰め (packing). The exact historical derivation is uncertain, but the word vividly conveys a sense of being tightly squeezed.