Green’s Dictionary of Slang

speculative.

1960		J.D. Macdonald Slam the Big Door (1961) 41: He was [...] putting up three low-cost spec houses.
1963		A. Baron Lowlife (2001) 20: It was probably named after the Victorian spec builder who ran it up.
1995		J. Stahl Permanent Midnight 68: You couldn’t slime into a 7-Eleven without the clerk bouncing a spec Cheers off you.
2004		J. Ellroy ‘Jungletown Jihad’ in Destination: Morgue! (2004) 333: He’s written some [...] spec scripts that caught my attention.