The invention relates to a fluid drain plug (10) for a fluid reservoir (50) with a drain hole (51), the plug having an extension in an axial direction (A) and comprising a body part (16) rotationally insertable into the drain hole (51), the body part having a proximal axial end (15), a distal axial end (17) and a substantially circular cross section with an outer surface (18), wherein the body part comprises a drain channel (92) having an inlet (91) located at the distal axial end and an outlet opening (94) disposed on the outer surface (18), the body part further comprising a bayonet-type connection slot (40) for accommodating a part of a radial projection (52) of a circumferential surface surrounding the drain hole (51), the bayonet-type connection slot being disposed on the outer surface and comprising an entrance (40a) at the distal axial end, a first path (41) and a second path (42), the first path (41) extending from the entrance, in the axial direction (A) and in one circumferential direction (C), to a first path terminal portion (70), wherein the first path terminal portion (70) is adapted to accommodate the radial projection (52), and the second path (42) extending from the first path terminal portion, in the axial direction (A) and in an opposite direction to the one circumferential direction (C), to a second path terminal portion (72), wherein the second path terminal portion (72) is adapted to accommodate the radial projection.