Lignify is a term used to describe the process of turning wood or other plant materials into lignin, essentially making them more woody or tougher. Some common synonyms for lignify include petrify, harden, or fortify, all of which convey a sense of increased durability. Other similar words include calcify, fossilize, and ossify, all of which suggest a hardening or solidifying process. Some more specialized synonyms for lignify might include evolve, mature, or develop, all of which suggest a process of growth or change over time. Ultimately, the choice of synonym will depend on the specific context and intended meaning.