Looking for a thick, syrup-like liquor...?!
Answers:
Nothing I can think of this may be impossible being that alcohol is thinner and with less viscosity than water and a liquor must be 40% or grater of alcohol.
If there is something it would be fairly uncommon and almost pure alcohol and sugar. Seeing ad a simple sugar is about 50/50 sugar to water mix. This could not be made this way as the temperature to dissolve that sugar would ignite the alcohol. but a thick syrup blended with a high proof alcohol would crate one.
Furthermore if you could get it VERY cold (colder than your standard freezer About -50 F or -45 C ) any alcohol will become viscose.
All depends on what your using the liquor for but grenadine syrup is good for cocktails. They use it for a sex on the beach cocktail and not sure on what other ones. This might mix well with rum but would mix the best with vodka id say.
Most liqueurs like Kahlua and Drambuie are thick, its the high sugar content, like maple syrup.
Jagermeister and Sambucca
The only thing I can think of is Bailey's, it's kinda thick, best I could do :)
interesting question
jagermeister