Weird gloop is a substance created as a result of not following the proper steps while creating magic glue during The Eyes of Glouphrie. It is created by using rune or pure essence on an anvil to create rune shards, grinding the shards to create rune dust, and then mixing the dust with a bucket of sap.
The error made when creating weird gloop is that mud runes, not rune or pure essence, should be ground to create the glue. The resulting bucket of weird gloop has no use, but can be emptied to retain the bucket. Creating weird gloop will result in the a game message in the chatbox stating "You mix the rune dust into the tree sap. It doesn't look quite right."