Green’s Dictionary of Slang

space (out) v.1

[one ‘flies’ into space]

(US) to daydream, to drift off, esp. when under the influence of drugs.

[US] ‘Sl. of Watts’ in Current Sl. III:2.
[US]Eble Campus Sl. Mar. 7: space – to be insensitive to what is happening.
[UK]T. White Catch a Fire 251: Rattled West Indian immigrants swore off ganja [...] convinced they had somehow spaced out and missed a year of Wailers 45s.
[US]S. Frank Get Shorty [film script] Yo, Chili, you’re spacin’.
[US]Mad mag. June 37: Man, I totally spaced.
[UK]L. Theroux Call of the Weird (2006) 34: ‘Oh yes,’ he said, recovering himself. ‘I remember. I’m sorry, I spaced out a little there.’.
[US]M. Lacher On the Bro’d 299: Then he like just spaced out and checked his phone.