  A well-known precursor of an imminent solar eruption is the appearance of a
hot S-shaped loop, also known as sigmoid, in an active region (AR).
Classically, the formation of such an S-shaped loop is envisaged to be
implemented by magnetic reconnection of two oppositely oriented J-shaped loops.
However, the details of reconnection are elusive due to weak emission and
subtle evolution during the pre-eruptive phase. In this paper, we investigate
how a single J-shaped loop transforms into an S-shaped one through the slippage
of one of its footpoints in NOAA AR 11719 on 2013 April 11. During an interval
of about 16 min, the J-shaped loop slips through a low-corona region of strong
electric current density in a bursty fashion, reaching a peak apparent speed as
fast as over 1000 km s$^{-1}$, at the slipping footpoint. The enhancement of
electric current density, as suggested by non-linear force-free field modeling,
indicates that the "non-idealness" of coronal plasma becomes locally important,
which may facilitate magnetic reconnection. The loop segment undergoing
slipping motions is heated; meanwhile, above the fixed footpoint coronal
emission dims due to a combination effect of the lengthening and heating of the
loop, the latter of which is manifested in the temporal variation of dimming
slope and of emission measure. These features together support an asymmetric
scenario of sigmoid formation through slipping reconnection of a single
J-shaped loop, which differs from the standard tether-cutting scenario
involving a double J-shaped loop system.
