1
a
: the reddish brittle coating formed on iron especially when chemically attacked by moist air and composed essentially of hydrated ferric oxide
b
: a comparable coating produced on a metal other than iron by corrosion
c
: something resembling rust : accretion
2
: corrosive or injurious influence or effect
3
: any of numerous destructive diseases of plants produced by fungi (order Uredinales) and characterized by usually reddish-brown pustular lesions
also
: a fungus causing this
4
: a strong reddish brown
rusted; rusting; rusts
1
: to form rust : become oxidized
iron rusts
2
: to degenerate especially from inaction, lack of use, or passage of time
Most men would … have allowed their faculties to rust.—T. B. Macaulay
3
: to become reddish brown as if with rust
the leaves slowly rusted
4
: to be affected with a rust fungus
1
: to cause (a metal) to form rust
Keep up your bright swords, for the dew will rust them.—William Shakespeare
2
: to impair or corrode by or as if by time, inactivity, or deleterious use
Years of storage in the damp basement rusted the tools.
3
: to cause to become reddish brown : turn the color of rust
Cite this Entry
“Rust.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/rust. Accessed 14 Aug. 2026.
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