knot
n.
Old English cnotta "intertwining of ropes, cords, etc.," from Proto-Germanic *knuttan- (cognates: Low German knütte, Old Frisian knotta "knot," Dutch knot, Old High German knoto, German Knoten, perhaps also Old Norse knutr "knot, knob"). Figurative sense of "difficult problem" was in Old English (compare Gordian knot). Symbolic of the bond of wedlock, early 13c. As an ornament of dress, first attested c.1400. Meaning "thickened part or protuberance on tissue of a plant" is from late 14c.The nautical unit of measure of speed (1630s) is from the practice of attaching knotted string to the log line. The ship's speed can be measured by the number of knots that play out while the sand glass is running.
The distance between the knots on the log-line should contain 1/120 of a mile, supposing the glass to run exactly half a minute. [Jorge Juan and Antonio de Ulloa, "A Voyage to South America" 1760]
Hence the word knot came also to be used as the equivalent of a nautical mile (in pre-WWII use in U.S. and Britain, 6,080 feet). A speed of 10 knots will cover ten nautical miles in an hour (equivalent to a land speed of about 11.5 mph).v.
"to tie in a knot," mid-15c., from knot, n.. Related: Knotted (late 12c.), knotting.