The Spanish verb 'preñar' (to impregnate) comes from the Vulgar Latin 'praegnare' meaning 'to make pregnant.' This Vulgar Latin term developed from the Classical Latin 'praegnas' meaning 'pregnant,' which was formed by combining two Latin elements: the prefix 'prae-' meaning 'before, in front of' and 'gnasci' meaning 'to be born.' The literal sense was thus 'before being born' or 'carrying before birth.'

The evolution from Latin to Spanish shows typical sound changes, where the Latin 'prae-' simplified to 'pre-' and the '-gn-' cluster evolved into the Spanish palatal nasal 'ñ', giving us the modern form 'preñar.'

Some related Spanish words include 'preñada' (pregnant), 'preñez' (pregnancy), and the more formal term 'embarazar' (to make pregnant). While 'preñar' is considered somewhat direct or crude in modern Spanish, these related terms share the same Latin roots and are used in different registers of the language.

While English doesn't have any direct cognates of 'preñar', we can find some distant relatives through the Latin 'prae-' prefix in words like 'precede' (to come before), 'predict' (to say before), and 'prefix' (to fix before). The 'gnasci' element is related to English words like 'natal' (relating to birth), 'native' (born in a place), and 'nature' (that which is born or produced).