gradually, slowly, deliberately, gently
徐 centers on unhurried, measured movement or action: doing something gradually, slowly, deliberately, or gently without rushing.
徐 combines 彳 (movement, going) with 余, which likely contributes the sound and may suggest a relaxed, unhurried quality. The exact historical role of 余 is uncertain.
The left side 彳 suggests walking or moving, and the right side 余 looks like a relaxed person under a roof. Picture someone strolling slowly and deliberately, taking their time.
For ジョ, imagine a slow, gentle Joe who never rushes: Joe -> ジョ, and he walks gradually along the path.
gradually; steadily; slowly; little by little; step by step; by degrees
suddenly; abruptly
going slowly (esp. to enable a vehicle to stop immediately); going at a reduced speed; slowing down; reducing speed
slow; gradual; steady
annealing; slow cooling
walking slowly
non-REM sleep
bradycardia; infrequent pulse
driving at reduced speed; running at reduced speed (e.g. train)
going at a very slow speed
gentle and quiet
adagio