Three wavy lines of rising vapor: breath, steam, air. From physical breath it expanded to weather (天气), temperament (脾气) and the life-energy 气 that flows through tai chi and Chinese medicine.
生气 to get angry is literally 'generate qi' — anger as pressure building inside. 天气 weather, 客气 polite ('guest air'). The same 气 is the qi of qigong and kung-fu films: breath as life force.
Wavy lines of rising STEAM — AIR, breath, and boiling ANGER.
不客气 'you're welcome' literally means 'don't be guest-air-y' — drop the polite formality.