As the episode synopsis says ''With war raging all around him, we get a close look at Negan and the lives of the Saviors during the conflict through a familiar set of eyes.'', my guess is that 'big scary U' refers to Negan. 'U' is 'you' and I think it might come from the mouth of Gabe when he and Negan has a dialogue when they are trapped in that room together. I further speculate that Gabe is trying to remove the mask of Negan by saying something like ''Under that big scary you, I can see you are just as insecure as anyone else. You are just putting up a scary front to hide your own fear and to control others.''

And, who is/are that ''familiar set of eyes''? Judging from the preview, I think it includes Dwight and Eugene. Temporarily without Negan, it looks like there is an early power struggle among the lieutenants.

 

I feel like the title is some weird thing Eugene would utter...

I imagine Dwight might get Eugene in on his double agent action plan and probably calls Eugene out for something afterwards, like being a coward even after being solicited to help his friends and bring down Negan...

Eugene might say something like "I am not like you, I am not a graduate of the big scary U" as in university, which could be the sanctuary or maybe the zombie apocalypse in general.

 

During their fight Rick to Daryl waving his hands in a mocking manner

and sarcastically calling Daryl "the big scary u"

 

I think it’ll be Gregory talking about the Unknown.

 

I imagined the Big Scary U was Negan referring to Gabriel. Gabe was sort of boasting that he was at the outpost that night and taking part in the murder of Negan's men.