The English phrase: to take a cup too much, to have a drop too much = in German: zu tief ins Glas schauen / gucken. This German expression means literally that one looks too deep into the glass, in other words that the person is drunk, has had too much alcohol.
After three glasses of beer, Mary couldn’t walk straight any longer because she had had a drop too much. = Nach drei Gläsern Bier konnte Mary nicht mehr gerade laufen, weil sie zu tief ins Glas geschaut hatte.
The guys are having quite a hangover today because they took a cup too much last night. = Die Jungs haben heute einen ziemlichen Kater, weil sie gestern nacht zu tief ins Glas geschaut haben.
Origins of this German expression:
The exact origins of the German idiom are unknown. However, linguists assume that ‘zu tief ins Glas schauen’ is actually related to the expression ‘jemandem zu tief in die Augen schauen’ (to look to deep into sb.’s eyes, meaning: to fall in love). So, in this case this means that the person ‘falls in love’ with alcohol.
By the way, this idiom has a bunch of similar expression that all mean the same. You may come across:
zu tief in den Becher gucken / schauen = to look too deep into the mug
zu tief in die Flasche gucken / schauen = to look too deep into the bottle
die Nase zu tief ins Glas stecken = to put the nose too deep into the glass
Here is what happens when German ‘look too deep into the glass’:
A clip from the famous movie “Feuerzangenbowle” with Heinz Rühmann