«تَعَارَجْتُ لَا رَغْبَةً فِي الْعَرَجِ، وَلَكِنْ لِأَقْرَعَ بَابَ الْفَرَجِ»
I affected a limp, not because I wished to be lame, but to knock at the door of relief.
Taʿārajtu means to affect a limp. Lā ... wa-lākin rejects one motive and substitutes another: not a desire for disability, but a quest for relief.
The purpose clause li-aqraʿa explains why the disguise was adopted; bāb al-faraj is a figurative door of relief. After this confession the apparent disability should no longer be treated as verified medical fact.
The denied motive and the affirmed motive must be distinguished.