An amalgamation of "Best Friend" and "Significant Other", usually referring to someone of the opposite sex. Essentially someone who you have no romantic connection to but still love (as a friend) anyhow - a step above "Best Friend" which is used too interchangeably with "Friend".
You know, you're not just a friend Alyssa, I'd call you a significant friend :-)
by monsieurmandarin July 17, 2021
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1. that other someone that a person in a relationship is diddling on the side; the dirty mistress, if you will.
2. synonym for the "free-pass fuck," that hot person or celebrity you would want to do and not have consequences while in a relationship.
Yeah, David is my boyfriend, but Harry is my significant nother.

Oh, yeah, Kevin knows that if Jake Gyllenhaal comes to town, he better not come back to the room, cause Jake is my significant nother.
by Siena K February 21, 2008
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A significant other whose status is not created by a agreed-upon relationship status but by how much time is spent with them. Could also be considered a behavioral significant other.

Combination of "statistically significant" and "significant other."
Since he spent twice as much time with Karen as with any of the other girls he was dating, his friends called Karen his statistically significant other.
by Brett Lider February 5, 2009
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A significant other defined by an (undesired) medical condition, not by a agreed-upon relationship status.

Combination of "clinically significant" and "significant other."
They were only dating casually, but when he gave her the Clap, he became her clinically significant other.
by Brett Lider February 5, 2009
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A person with whom one shared a special relationship that is somewhat more than simply as friend, but not not quite the level of "significant other." May be former or future significant other, or even someone whom will never have that status. Some "best friends" qualify, but not nearly all do. A sexual component to the relationship is quite common, though it is not required.

Note: A sexual component does not automatically raise a friend to moderately significant other status, rather it is the strength of the bond between the two parties.
"I haven't had a girlfriend in a while, so I've been spending a lot of time with my moderately significant other, and we've even gone to a few weddings together. But somehow, we've never quite gone out."
by ceolaf2 September 1, 2009
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A person you are dating casually, but is potentially a keeper.
I thought you and Kimberly were monogamous.
Well, she is a semi-significant other, but we haven't committed to anything yet, so I'll keep trolling while I wait for her to get serious.
by Obscure One November 18, 2016
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