WordNet
n. a protein gelatin obtained by boiling e.g. skins and hoofs of cattle and horses
Wikipedia
An animal glue is an adhesive that is created by prolonged boiling of animal connective tissue.
These protein colloid glues are formed through hydrolysis of the collagen from skins, bones, tendons, and other tissues, similar to gelatin. The word "collagen" itself derives from Greek κόλλα kolla, glue. These proteins form a molecular bond with the glued object.
Stereotypically, the animal in question is a horse, and horses that are put down are often said to have been "sent to the glue factory". However, other animals are also used, including rabbits and fish.
Usage examples of "animal glue".
We called him 'Black Peter' and when we went to watch him working he'd sometimes let us help him with sandpapering or boiling up the smelly animal glue.
The angle of light is direct and severe, making the people on the bed appear to us in a special framework, their intrinsic form perceivable apart from the animal glue of physical properties and functions.
The batteries were so gummed up by the cold that they couldn't even have turned over a hot engine, far less one in which the crankcase, transmission and differential were all but locked solid in lubricating oil that had lost all power to lubricate anything and had been turned into a super viscous liquid with the consistency and intractability of some heavy animal glue.