Is there anywhere in business to store luggage in Washington D.C. during a layover

Are you flying into Reagan? If on one ticket, just check your luggage through. But am guessing you are not on one ticket. My best bet would be to use a hotel. Do you have any status with any of the chain hotels? I suppose you could wing it a bit, check you luggage with a hotel by implying that you are checking in later. Tip the bellman well.

Try "https://stasher.com/". They have locations in Washington, D.C.

My flights are separate tickets arriving 9am Reagan, (living it up at the museums during the layover) leaving for Munich (yay) from Dulles at 10pm.

I used the "https://stasher.com/". link to search stasher and when I could not find a location wrote them and they just replied
"Hi JazzTravels, unfortunately we do not have a location in Washington DC yet! Within a few months though we should do :)

Rats. I was not planning to go to any of those museums.
DC like New York never strikes me as a place with locals anymore. It is all transplants.

All of the museums are going to inspect your bag, if they have storage. There is a checkroom at the National Gallery of Art and it looks like they can handle a carry-on sized bag.
https://www.nga.gov/visit/visitor-policies.html

If you are interested in going to the Udvar-Hazy, they have lockers. You could head directly to Dulles and then to the museum
https://airandspace.si.edu/visit/udvar-hazy-center/visiting-tips

You might also check prices of day use hotel rooms.

Will you be here on a weekday or weekend?

signed, a D.C. Native

Also, this page lists which of the Smithsonian Museums have lockers:
https://www.si.edu/visit/tips

I'm afraid this probably won't help you, given your schedule, but I'll throw it out anyway. There are luggage storage lockers at the Udvar-Hazy Center near Dulles. This is an annex of the Smithsonian Air and Space Museum. It has a fantastic collection of historic aircraft and many people visit it during a layover at Dulles. The museum is about 10 minutes from the airport, with regular shuttles operating between.

https://airandspace.si.edu/udvar-hazy-center

I guess Laura and I were typing at the same time. :-)

Ok I feel better thanks for the info. I am there on a Tuesday in two weeks

It isn't Washington specific, and maybe given the security requirements it won't suit, but there are storage alternatives popping up...like bike shops, or laundromats. Some apps to also locate storage options.
Here is an NYTimes article
https://www.nytimes.com/2018/03/02/travel/getaway-luggage-storage.html?smid=fb-nyttravel&smtyp=cur