The Collaborative International Dictionary of English v.0.48:
Thick \Thick\, n.
1. The thickest part, or the time when anything is thickest.
In the thick of the dust and smoke. --Knolles.
2. A thicket; as, gloomy thicks. [Obs.] --Drayton.
Through the thick they heard one rudely rush.
He through a little window cast his sight
Through thick of bars, that gave a scanty light.
Thick-and-thin block (Naut.), a fiddle block. See under
Through thick and thin, through all obstacles and
difficulties, both great and small.
Through thick and thin she followed him. --Hudibras.
He became the panegyrist, through thick and thin, of
a military frenzy. --Coleridge.