The largest English dictionary out of copyright is the Century Dictionary. Most of the 20th c dictionaries freely copied from it. There are many online resources derived from it at various levels of freedom. For example, http://www.global-language.com/CENTURY/
There is also the Baldwin Dictionary of Philosophy and Psychology: https://archive.org/details/dictionaryphilo00baldgoog/page/n5
There are also versions derived from it, such as https://psychclassics.yorku.ca/Baldwin/Dictionary/
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