The Spanish verb form 'cierro' (meaning 'I close') comes from the infinitive 'cerrar' ('to close'), which can be traced back to the Vulgar Latin 'serrare' meaning 'to close'. Going further back, 'serrare' developed from the Classical Latin 'serare' meaning 'to bolt' or 'to lock', which was derived from the Latin noun 'sera' meaning 'bolt' or 'lock'. The evolution from 's' to 'c' at the beginning of the word is a common sound change that occurred as Latin evolved into Spanish. The 'ie' in 'cierro' is due to a regular Spanish vowel change where 'e' becomes 'ie' in stressed syllables.

Some related Spanish words include 'cierre' (closure, closing), 'cerradura' (lock), and 'cerrado' (closed). All these words share the same Latin root 'sera' (bolt, lock) and maintain the basic concept of closing or securing something.

While English doesn't have many direct cognates from this Latin root, we do see it in some borrowed words like 'serre' (a greenhouse, from French, ultimately from the same Latin root, as it's an enclosed space) and 'serried' (pressed or crowded together, as in 'serried ranks'). The connection is that these words relate to things being closed, pressed, or locked together.