The Spanish word 'hermanito' meaning 'little brother' comes from combining the Spanish word 'hermano' (brother) with the diminutive suffix '-ito'. The word 'hermano' itself traces back to the Latin word 'germanus' meaning 'genuine' or 'natural', which ultimately derives from Latin 'germen' meaning 'germ' or 'sprout'. The semantic evolution from 'genuine' to 'brother' likely came from the phrase 'frater germanus' meaning 'genuine brother' (as opposed to a half-brother or step-brother), which was eventually shortened to just 'germanus'.

The diminutive suffix '-ito' is a very common Spanish ending that makes words smaller or more endearing. When added to 'hermano', it creates 'hermanito' - literally 'little brother', but also carrying an affectionate connotation.

The most obvious related Spanish word is 'hermano' (brother), which is the base word from which 'hermanito' is formed. Another common related word is 'hermana' (sister). The '-ito' suffix is extremely common in Spanish and appears in many everyday diminutives like 'perrito' (little dog), 'gatito' (little cat), and 'chiquito' (little one).

While English 'brother' is not etymologically related to 'hermanito', we do have some English words that come from the same Latin root 'germen'. These include 'germ' (in its meaning of a seed or beginning of something), 'germinate' (to sprout or begin to grow), and 'germane' (relevant or appropriate). The word 'germane' is particularly interesting as it comes from the same Latin 'germanus' as Spanish 'hermano', but while Spanish developed the meaning toward family relationships, English maintained more of the original sense of 'genuine' or 'naturally related'.