Consider the following cryptic crossword clue:
Sounds like an alarming poison. (6)
Experienced cruciverbalists will instantly recognize the “sounds like” as a clue element that suggests the answer is some sort of homophone – a word that sounds like another. The next step is therefore to come up with a word related to poison, and one such word is toxin. Finally, all that’s needed is to think of a word that sounds like toxin and which is also related to the concept of “alarming.” Bada bing, bada boom – it’s tocsin.
The original meaning is defined by the OED as;
A signal, esp. an alarm-signal, sounded by ringing a bell or bells: used orig. and esp. in reference to France.
The first written reference to the word comes from 1586 in William Fulke’s A Treatise against the Defense of the censure, giuen upon the bookes of W. Charke and Meredith Hanmer, by an unknowne popish traytor:
The priests then went vp into the steeple, and rang the bells backward, which they call Tocksaine, whereupon the people of the suburbs flocked togither.
By the mid-1800’s, the more general use of tocsin as a signal became more specific over time to refer directly to the actual bell used as opposed to the act of warning. In his poem, The Belfry of Bruges, Longfellow refers to the bell in the tower as “the tocsin“;
And again the whiskered Spaniard all the land with terror smote;
And again the wild alarum sounded from the tocsin’s throat;
Till the bell of Ghent responded o’er lagoon and dike of sand,
“I am Roland! I am Roland! there is victory in the land!” 
The word can be traced back to the French tocsin, which appears in Old French as toquassen and Provencal as tocasenh. It actually comes from two words; tocher meaning “to touch or strike” and senh=”a sign” or, in this case, “a signing bell.” Ultimately the Latin roots are *toccare=”to touch” and signum=”a sign,” which also was used in late Latin for bell.
Today it would be rare to refer to a tornado siren or a fire alarm, for example, as a tocsin even though it would be accurate. But feel free to give it a try next time the warning goes off (“Hark, yonder tocsin is a harbinger of meteorological peril!”)
 The bell in the Belfry of Ghent was named Roland after King Charlemagne’s nephew and Commander-in-Chief, Roland, who fought at the Battle of Roncevaux Pass in 778. His story is told in the 11th century poem, The Song of Roland, where the battle takes place between the Franks and the Spanish Muslims, resulting in Roland’s demise. During the fighting, as the Franks started to be overwhelmed, Roland used a horn called an olifant to call for help from the rest of Charelmagne’s army, hence the notion of a “warning” or “summons for aid.”