in the thick of

Jennifer WeinerのFly Away Homeを読んでいます。
When she was in the thick of her residency, working thirty-six-hour shifts, then going home to a preschooler and a husband who was more inclined to complain about his own lack of sleep than to help Diana address hers, there was a patient, a repeater, whom she saw every few weeks in the ER. The patient’s name was Crystal, and Diana remembered her because she was exactly her age.
"in the thick of" は文脈から何かの真っ最中、真っただ中の様な意味だと推測できますが、初めて見る表現なので辞書を見ます。
・Merriam-Webster's Online Dictionary: in the most active or intense part of (something): a soldier in the thick of battle
・Collins Dictionary: If you are in the thick of an activity or situation, you are very involved in it.: I enjoy being in the thick of things.
"residency" は以前(11/20/2013) "resident" で覚えた意味と関係のある単語ですね。