The Spanish word 'sencillito' meaning 'very simple' comes from 'sencillo' (simple) with the addition of the diminutive suffix '-ito'. The word 'sencillo' itself can be traced back to the Vulgar Latin 'singellus' meaning 'simple' or 'single', which in turn derives from the Classical Latin word 'singulus' meaning 'one at a time'. The diminutive '-ito' adds a sense of endearment or smallness, so 'sencillito' literally means 'a little simple' or 'very simple' in an endearing way.

The most obvious related Spanish word is 'sencillo', which means 'simple' or 'straightforward'. This is the base word from which 'sencillito' is formed. While 'sencillo' can also mean 'single' (as in a single dollar bill) or 'uncomplicated', its diminutive form 'sencillito' emphasizes the quality of being simple in a cute or pleasant way.

While there aren't direct English cognates that are obviously similar to 'sencillito', English words that share the Latin root 'singulus' include 'single' and 'singular'. You can see how the meaning evolved from the Latin 'one at a time' to both the English concept of 'single/singular' and the Spanish concept of 'simple/uncomplicated', as something that is singular or individual is often simpler than something multiple or complex.