A tocsin consists of a signal of alarm given by the ringing of a bell, and hence any warning or danger signal.
The earliest form of the word in English appears as toclesaine, borrowed from the Old French touque-sain (from toquer (to strike), compare toucher), combined with sin (modern French signe (a signal) from the Latin signum). The use of touch and its cognate forms with the idea of giving a sound also appears in the word tucket , Italian toccata, which probably originally meant a signal given by tap of drum, but subsequently always applied to a flourish or fanfare on a trumpet.
Original data from the 1911 Encyclopaedia Britannica