The Spanish word 'niña' (meaning 'girl') comes from the masculine form 'niño' (meaning 'boy'), which developed from Old Spanish 'ninno' meaning 'child'. This ultimately traces back to Medieval Latin 'ninnus', which was a nursery word meaning 'child' - similar to how we might use baby-talk words when speaking to young children. The feminine form was created by changing the final -o to -a, following the typical Spanish pattern for gender marking.

The most obvious related Spanish word is 'niño' (boy), which is extremely common and often among the first words learned by Spanish students. Together, 'niño' and 'niña' form a basic gender pair in Spanish. These words are also found in many common compounds and phrases like 'niñera' (babysitter), 'niñez' (childhood), and 'El Niño' (the weather phenomenon, literally meaning 'the boy child', referring to the Christ child since it often occurs around Christmas).

While there aren't direct English cognates from the same Latin root 'ninnus', English speakers might be familiar with 'niña' from historical contexts, particularly 'La Niña' (one of Columbus's ships) or from the weather pattern 'La Niña' (the counterpart to 'El Niño'). The word has also entered English usage in phrases like 'quinceañera' (celebration of a girl's 15th birthday).