IPA: //ˈhɑːrtʃɔrn//
KK: /ˈhɑrtʃɔrn/
A substance made from ammonium carbonate, often used in smelling salts, or the antler of a male deer, which was historically used to produce ammonia.
In the past, hartshorn was commonly used as a remedy for fainting spells.
Hartshorn is derived from the Old English "heorot" (meaning stag or deer) and "horn" (meaning horn). The term originally referred to the horn of a stag, which was used in various medicinal and practical applications.
Think of a 'stag' ('heorot') and its 'horn' — that's why hartshorn refers to the horn of a stag.