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An abbreviation of the term significant other, less unwieldy, used to refer to someone who is more than a boyfriend or girlfriend, but not yet a husband or a wife--usually a co-habitating lover, with whom one shares a long-term sexual relationship. May be also be used as a gender-neutral term of reference for a same-sex partner.
"This is my Signo, Zachary."

"Why don't you meet us for dinner, and bring your Signo if you'd like."
by Doctor Derek Faux February 21, 2008