the.com/square knot

means A simple knot made by tying two ends together with two opposite half-hitches, used to bind a single object or join two ropes of equal thickness — secure when static, unreliable as a bend under strain.

from A descriptive English compound: 'square' for the neat, symmetrical, four-sided shape the dressed knot makes, plus 'knot,' an old Germanic word (related to Dutch 'knot' and German 'Knoten'). It's also called the 'reef knot,' from its use reefing sails — gathering and tying down canvas to shorten it in heavy weather — because the flat knot lay snug against a spar and could be released quickly with a tug.