Edit
Yassai Mossai (Sokonoke Sokonoke) is the local dance of Kisarazu city of the Chiba prefecture.
Senki Zesshou Symphogear
From [1]:
'In the second OVA when Hibiki explains to Miku about the mission she and Chris are doing, we see thought bubbles with Chris shouting "Yassai Mossai" repeatedly, and later "Chossai" ...'
'... Yassai Mossai is the name of the representative dance of Kisarazu city in Chiba prefecture, Japan. Chris was taught this dance by her parents when she was young (there’s a picture of this in the manga!). “Yassai Mossai” are really just lyrics created to fit the song, but they do carry the meaning of “Get out of the way! Get out of the way!‘
'Chris’s other phrase, "Chossei” (ちょっせぇ) is a shortened version of “Chorokusai” (ちょろくさい) which in turn was a mix of the phrases “Choroi” and “Kusai.” It carries various meanings depending on the context, but it’s intended to be insulting. When directed at a person it can be “weak,” “dumb,” or “slow.” When referring to a task it can mean “ too easy” or “no big deal.".'
Links
1