The Spanish word 'espantoso' (meaning 'frightful' or 'horrible') has its roots in Latin. It comes from the Latin verb 'pavere' meaning 'to be frightened', combined with the prefix 'ex-' meaning 'out', which formed the Latin verb 'expaventare' meaning 'to frighten'. This evolved into the Spanish noun 'espanto' meaning 'fright' or 'terror'. The addition of the Spanish suffix '-oso' (meaning 'full of') to 'espanto' created 'espantoso', literally meaning 'full of fright' or 'frightful'.

The transformation from Latin 'expaventare' to Spanish 'espanto' shows typical sound changes: the 'x' became 's', and the middle portion of the word was simplified. The modern meaning maintains a clear connection to the original sense of fear and fright.

Some related Spanish words that might be easier to learn include the noun 'espanto' (fright, terror) from which 'espantoso' is directly derived. There's also the verb 'espantar' meaning 'to frighten' or 'to scare away'. These words share the same Latin root and form a family of words related to fear and fright in Spanish.

While English doesn't have direct cognates from the Latin 'pavere', we can find some distant relatives in words like 'panic' and 'panic-stricken', which come from a related root referring to fear. The suffix '-oso' in Spanish is cognate with English '-ous' as in 'dangerous' or 'perilous', both meaning 'full of' something.