Reluctance to Guess Definitions

assignment in question
I've encountered an interesting phenomenon with my Tribal Scholars students this year, based on one assignment I've given all of them: They seem very reluctant to guess.

The assignment was a vocabulary "pre-assessment" where I gave the students a list of assorted social studies vocabulary (ranging from "communism" to "ethnocentrism" to "Renaissance"). I asked the students to record whether they knew the word or not, and then write a definition for the word if they did, or guess what the definition could be if they didn't.

I would have supposed that writing definitions to words they knew but couldn't fully explain would be the hardest part of the assignment. Instead, many of the students were extremely reluctant to guess what the definitions of words they didn't know could be. I thought it would be easy to look at a term like "settler colonialism" and make a guess like "people setting up a colony," but the students didn't want to write anything at all. Eventually I just asked them to write down whatever came to their mind on seeing the word, (which led to things like "Star Wars" as a guess for "imperialism"), but there were still many who didn't want to write.

I wonder what was behind this widespread reluctance to guess definitions—general discomfort with words? a lack of imagination? hesitance to not have the "right answer"? In any case, I don't know the answer, but I just thought I'd share these observations and see if other teachers out there have seen this same phenomenon.