The team played well in the first half but completely tanked in the second half.

tanked 是动词 tank 的过去式。tank 原指"坦克"或"水箱"，在俚语中引申为"彻底失败"或"表现极差"，类似于"崩溃"或"垮掉"的意思。这里形容球队下半场表现突然变得非常糟糕。