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1.
This is when a girl gives oral sex on a soft penis. It does not refer to continuing to suck after ejaculation, but sucking the soft penis from start to finish. She will swallow all the pre-cum, but a full ejaculation is not likely to occur.
I can't get hard because this girl is a trash bag titty whore, but I'd still let her soft blow me.
by esteban de queen February 20, 2011
 
2.
Failing to achieve erection before ejaculation. Often painful and leads to feelings of shame and inadequacy.
The experience of wearing a G-string caused Derek to soft-blow. What a n00b....pwned!
by Giner December 23, 2003