for standard words, you can try gaming, like scrabble/boggle/etc but those remove words that are not legal, too long, unpossible with the game tiles, etc.
you can get the whole english dictionary from miriam webster as a text file.
past that you are in for some legwork, eg you might use this:
and extract it as text and run a little parser program over it to reformat it into what you need. I don't think most of those will come in a 'dictionary' text file format, but you can build one easily from various online resources, faster than you can get a better answer from a bunch of people :) (there may be a better answer, but its only like 15 min to build one too).
cmd is very small and you could start with a redirect to text file of the help and parse out the words. There are probably some undocumented commands from dos days that are not in the help, but it would be a solid starting point.
notepad ++ has dictionaries for a LOT of languages. I think they are just text files and I think you can use those as well, eg for c++ keywords. For example, under langs.xml you can see this for c++ (partial, I cut it off)
<Language name="cpp" ext="cpp cxx cc h hh hpp hxx ino" commentLine="//" commentStart="/*" commentEnd="*/">
<Keywords name="instre1">alignof and and_eq bitand bitor break case catch compl const_cast continue default delete do dynamic_cast else false for goto if namespace new not not_eq nullptr operator or or_eq reinterpret_cast return sizeof static_assert static_cast switch this throw true try typedef typeid using while xor xor_eq NULL</Keywords>
<Keywords name="type1">alignas asm auto bool char char16_t char32_t class clock_t concept const consteval constexpr constinit decltype double enum explicit export extern final float friend inline int int8_t int16_t int32_t int64_t int_fast8_t int_fast16_t int_fast32_t int_fast64_t intmax_t intptr_t long mutable noexcept override private protected ptrdiff_t public register requires short signed size_t ssize_t static struct template thread_local time_t typename uint8_t uint16_t uint32_t uint64_t uint_fast8_t uint_fast16_t uint_fast32_t uint_fast64_t uintmax_t uintptr_t union unsigned virtual void volatile wchar_t</Keywords>
and there is one for about 50+ languages.