The term was heard by this writer in 1977-1978 at Fort Bragg, NC, post-Vietnam era. It was considered the highest compliment that could be paid by one soldier to another. It was always given to male soldiers, as female soldiers were so few at the time and it tended to be a "guy thing." It might derive from one of a number of related words in old German, Dutch, or English meaning "tight," "tense," or "strict" (see "strak" in Wiktionary-dot-org).
"Man, you are strack!" "Hey, thanks."