The Spanish word 'snack' is a direct borrowing from English 'snack', which traces back to Middle English. In Middle English, 'snacchen' meant 'to bite', and the related noun form 'snack' meant 'a bite or morsel'. Over time in English, the meaning evolved from just a bite to refer to a light meal or small amount of food eaten between regular meals. When the word was borrowed into Spanish, it retained this modern meaning of a light meal.

Since 'snack' is a relatively recent borrowing from English into Spanish, there aren't any etymologically related simpler Spanish words. The concept of a light meal or snack in traditional Spanish might be expressed with words like 'bocadillo', 'tentempié', or 'merienda', but these aren't etymologically related to 'snack'.

The English word 'snack' is obviously related, being the direct source of the Spanish borrowing. The English verb 'snatch' is also etymologically related, as both 'snack' and 'snatch' come from Middle English 'snacchen'. You can see how the quick, grabbing motion implied in 'snatch' relates to the quick bite or quick meal implied in 'snack'!