Earth-abundant electrocatalysts for stable hydrogen generation in acidic conditions at low overpotential are highly desirable and yet extremely challenging due to the corrosive degradation of the catalysts at low pH (<= 2). Here, we demonstrate gas-phase, thermally-driven selective oxidative tuning of structure and morphology of bulk Ti3AlC2 (MAX phase) to yield (a) randomly oriented polycrystalline Ti0.2Al1.8C4O5 nanowires (TACO NW) or (b) oriented single crystalline Ti0.2Al1.7C5O5.3 nanorods (TACO NR). The polycrystalline TACO NW exhibits high electrochemical surface area (0.11 cm2) and low overpotential (248 mV) for hydrogen evolution reaction (HER) compared to TACO NR, in acidic conditions, highlighting an important, structure-property correlation. The mechanism of nanostructuring and its subsequent electrocatalytic pathway is presented. Consequently, the dimensionally confined TACO NW provides high kinetic facility (Tafel slope = 121 mV/dec) and low chargetransfer resistance (70 ohm), resulting in stable HER with high Faradic efficiency (98 %) and turnover frequency (1080 h-1) at mass-activity (71,420 mA/g) that is significantly superior compared to several other similar systems.