three people sleeping side by side; sleeping like the character 川
Refers to the arrangement of three people lying side by side, typically a child between two parents, resembling the three strokes of the kanji 川. Used in set phrases like 川の字で寝る or 川の字になって寝る.
The three of us in the family slept side by side like the character 川.
When the kids were small, we often slept three in a row.
From the visual resemblance of three people lying side by side to the three parallel strokes of the kanji 川 (river).