Undrinkable describes a liquid that cannot or should not be drunk, usually because it tastes terrible, smells awful, or could make you sick. Water from a polluted pond is undrinkable. Milk that's gone sour and chunky is undrinkable. Coffee that sat on the counter for three days is undrinkable.

Sometimes people use the word more loosely to mean something tastes so bad they refuse to drink it, even if it wouldn't actually harm them. Your little brother might declare his medicine undrinkable because it tastes bitter, or you might find cafeteria milk undrinkable after it's been sitting in a warm lunchbox all morning.

The word reminds us that not all liquids are safe or pleasant to drink. Ocean water is undrinkable because the salt would make you sick. Industrial chemicals are undrinkable. Even tap water can become undrinkable if pipes break or contamination occurs, which is why communities issue “boil water” notices after floods or equipment failures.

Scientists and aid workers focus heavily on making sure people everywhere have access to drinkable water, since undrinkable water can spread disease. Clean, safe drinking water is one of humanity's most basic needs.