In Spanish, ‘leche’ means “milk”; “gatas,” in Filipino.
There have been a number of suggested origins of how this word has become a curse word. Here’s one of those, just a personal conjecture based on my familiarity with the circumstances surrounding the subject matter.
During the Spanish colonial period in the Philippines, many Spanish families employed local Filipinos as nannies or househelpers. Of course, these Spaniards spoke Spanish; and the locals spoke their local language or dialects and did not understand Spanish; because, apparently, their lords did not really teach them the details of the language.
Every time the Spanish family members spoke, the Filipino nannies simply listened. Then, whenever the babies cried and the parents wanted them to stop crying, they commanded the nannies to “give milk to the baby” to make the baby stop crying. In Spanish, “Give milk to the baby” translates to ‘Dar leche para el bebé!’
The emphasis was on the word ‘leche,’ giving the nanny an impression that it was a curse word because the parent would usually utter this sentence with an imposing tone of voice. The nanny thought that her employer was mad at her and was cursing her. Now, the nanny, upon returning to her own home, when she got upset, she started using ‘leche’ as an expression of her own frustration or annoyance. Through time, the word, in that context, seeped into the lexicon or lingo of the Filipino people.
The Last Leaf
Sa susunod na gamitin o marinig mo ang salitang iyan, alam mo na ang isa sa malamang na pinanggalingan nito. “Ah, gatas lang pala.” Subalit, hindi pa rin yan dahilan upang murahin mo ang taong iyong kinaiinisan. Anu’t anuma’y mas maige pa rin ang pagkamahinahon kaysa mabilis na pag-init ng ulo.