When you do so much good (helping others, donating to charity, ETC.) that it actually excites you.
"I've done so much good... I have a soul boner." -- Neil Patrick Harris from How I Met Your Mother.
The technical description for what's otherwise known as "warm and fuzzy" or "spiritually lascivious;" generally a "deep down inside" kind of feeling that expresses delight and gratification. Basically, it's a boner that both women and men of ALL ages can acquire at any moment in time. This is a unique and distinguishing feature of the soul boner, as it is the only boner available on demand (barring a talented minority of males). Soul boners can be caused by a large portion of the emotional spectrum, spanning from excitement (e.g Ricky Bobby) to amazement (e.g. Shawn Johnson) to downright badassness (e.g. Inglorious Basterds).
Soul boners are commonly known to occur at rock concerts, theme parks, and sporting events. Less commonly, they can occur at the zoo, in church, or in outer space.
Jim: "Hey, Kyle's finger-launching that bridesmaid by the reflecting pool!"
Nick: "Fuck yeah. That shit gives me a boner."
Bob: "That is inappropriate, Nick. Kyle is our friend."
Nick: "Don't be a fag, Bob. It's just a soul boner. I'm rooting for Kyle, that bitch is a MILF!"
(noun)an unexplainable erection achieved during church services
(adj) That little, warm fuzzy feeling you experience when you see something adorable
(N) Preacher: Praise Jesus!
Man: (achieves erection) Praise indeed!
(adj)(Watching tv, Snuggle commercial comes on)
person 1:Aw!!! Look at the Snuggle bear!
person 2: I might have a soul boner right about now