An Etymological Dictionary of Astronomy and Astrophysics

1. Capable of burning, corroding, or destroying living tissue. A caustic substance.
   

2. Optics: The enveloping surface formed by light rays reflecting or refracting from a curved surface. → caustic curve, → caustic surface.

Etymology (EN): M.E., from O.Fr. caustique, from L. causticus “burning,” from Gk. kaustikos “capable of burning,” from kaust(os) “combustible,” from kaiein “to burn” + -ikos, → -ic.

Etymology (PE): Sucân, from suc- “to burn,” variant of suz-, suzidan, suxtan “to burn;” cf. Baluci suc-, soc-;
Mid.Pers. sôxtan, sôzidan “to burn;” Av. base saoc- “to burn, inflame” sūcā “brilliance,” upa.suxta- “inflamed;” cf. Skt. śoc- “to light, glow, burn,” śocati “burns,” (caus.) socayati, śuc- “flame, glow,” śoka- “light, flame;” PIE base *(s)keuk- “to shine.”