Abarim Publications' online Biblical Greek Dictionary
The noun σειρα (seira) means cord or rope: the thing to tie something up with, most specifically ending in a noose or lasso. When the horse culture emerged from the Asian steppes, the noun σειραφορος (seiraphoros) could describe a wrangler (he who carries a lasso), or a trace-horse (he who is bound to the horse that's doing the actual pulling). In the New Testament our noun occurs only once, namely in 2 Peter 2:4, where it is used to describe bindings of darkness.
The familiar name Sirius, of the proverbial "Dog" of Orion, derives from the Greek Σειριος (seirios), which in turn comes from our noun σειρα (seira) and means the Bound One.
It's not clear where our word comes from, but some propose a connection to the verb ειρω (eiro), which means to bind in rows (of beads in an ornament) and which is spelled identically to the verb ειρω (eiro), to say or ask, hence the verb ερομαι (eromai), which became ερωταω (erotao), to ask. The leading σ might be explained as a remnant of the preposition εις (eis) meaning in, to or toward, which is a mechanism that may also explain the link between the names Selloi and Selene and Hellen and Helen (see our article on the name Hellas for a lengthy look at this), and may even explain words like σεβας (sebas), reverential awe, from the verb σεβομαι (sebomai), to revere, as relating to the noun βασιλευς (basileus), meaning king, or the similarity between the verb στρεφω (strepho), to turn, and the verb τρεπω (trepo), to turn (not to be confused with τρεφω, trepho, to raise up), and the verb τελλω (tello), to accomplish, and στελλω (stello), to put or set (see ανατελλω, anatello, to rise).
Others propose instead that our word was imported into Greek from another language. That other language would then most likely be Semitic — since the Greek alphabet is an adaptation of the Semitic one, and was most probably introduced into the Greek speaking world along with native Semitic words that described the rudiments of complex thought — which brings our noun σειρα (seira) in close phonetic proximity of names like Sarah and Seir.