On free and open-source systems, such as Debian Linux, install dict-* packages, such as dict-gcide with the community-maintained version of Webster’s 1913 dictionary or dict-moby-thesaurus Moby Thesaurus II.

The macOS jail, with its Homebrew, does not seem to offer any dictionary packages for dictd (RFC 2229), so use the Internet.

(with-eval-after-load 'dictionary
  (setopt dictionary-server "dict.org"))

Experiment.

Use the familiar Help mode for the interface.

Note. This does not work quite yet; reported:

notmuch-show:m2pm6nz5n0.fsf@me.com

(with-eval-after-load 'dictionary
  (setopt dictionary-search-interface 'help))