Why is the ninth month called September?
mid-13c., verbal noun from bind (v.). Meaning "thing that binds" is from c.1300; "state of being bound" is from late 14c. Meaning "covering of a book" is recorded from 1640s.
Old English bindan "to tie up with bonds" (literally and figuratively), also "to make captive; to cover with dressings and bandages" (class III strong verb; past tense band, past participle bunden), from Proto-Germanic *bindan (cf. Old Saxon bindan, Old Norse and Old Frisian binda, Old High German binten "to bind," German binden, Gothic bindan), from PIE root *bhendh- "to bind" (see bend). Intransitive sense of "stick together" is from 1670s. Of books, from c.1400.
"anything that binds," in various senses, late Old English, from bind (v.). Meaning "tight or awkward situation" is from 1851.