This paper proposes an adaptive output feedback event-triggered tracking control scheme for a class of nonlinear systems with severe nonlinearities and output constraints, in which the severe nonlinearities consist of output based nonlinear functions and unmeasurable states. To estimate the unknown state variables and compensate for the severe nonlinearities, a dynamic gain is first designed using gain scaling technique, based on which a modified high-gain K-filter is constructed. Then, an adaptive dynamic surface output feedback controller is designed by introducing the prescribed performance for the tracking error. Further, a triggering condition is constructed to determine the event-triggered control signal so that communication resource can be effectively saved. It is shown that with the proposed scheme, the severe nonlinearities and the execution error can be accommodated, the output constraints can be guaranteed, and the tracking error can converge to an arbitrarily small residual set. Finally, the effectiveness of the control scheme is demonstrated through simulation on a hypersonic aircraft.