When you are attracted to a girl, but only as a friend either because she practices chastity or does not want to have sex before she marries. Can also be used for items that you like, but you don't love.
I have such a soft-on for Emma, but she only wants to be a friend.
a "soft-on" is when the male penis fills with blood but not enough that would make the penis begin to get erect. The penis swells - but there's no visible elevation yet.
I was thinking about this babe at school and the teacher called me to the chalkboard. Luckily i only had a soft-on so I could stand up and walk to the board without being embarrassed.