Wiktionary
old sweat
alt. 1 (context British military slang esp. WWI English) An experienced soldier. 2 (context British military slang English) A veteran soldier or war veteran. 3 (context British slang figuratively English) Someone experienced in his field. n. 1 (context British military slang esp. WWI English) An experienced soldier. 2 (context British military slang English) A veteran soldier or war veteran. 3 (context British slang figuratively English) Someone experienced in his field.