Fig.: to be in such a crowded place that you can barely move

I hate the subway at rush hour because we travel packed like sardines

Odio viaggiare all'ora di punta sulla metropolitana perché si viaggia pigiati come sardine

I tried to get on the last bus before the strike started, but it was already full of people packed in like sardines and I couldn't get on

Ho provato a salire sull'ultimo bus prima che cominciasse lo sciopero, ma era già strapieno di gente pigiata come sardine e non ci sono riuscito

Salted sardines were often the food for sailors and armies. To make their transportation easier, the sardines were pressed on salt layers inside wooden barrels.
Today the expression alludes to the way in which sardines are packed closely together during canning.