What does a fifth of alcohol mean in terms 5 fifths equals what?

"A fifth" of alcohol refers to a fifth of a gallon, which is equal to 25.6 fluid ounces.

Therefore, 5 fifths of alcohol equals:

5 * 25.6 fluid ounces = 128 fluid ounces

This is equivalent to 4 standard bottles of liquor (since a standard bottle of liquor is 32 fluid ounces).