means Joining metal pieces by melting a softer filler metal (solder) between them, which cools into a solid electrical or mechanical bond without melting the base parts themselves.

from From the verb 'solder,' which came into English through Old French 'souder' (to consolidate, make solid), from Latin 'solidare' (to make firm), a relative of 'solidus' — solid. The buried 'l' that we now sometimes pronounce was actually restored by scholars looking back to the Latin; for a long stretch English speakers happily said 'sodder,' a pronunciation that still survives in American English.