Since Twelfth Night was written, many words in English have changed their meaning, and some are no longer used. If you remember the slang you used a few years ago, it seems dated. Who now uses the word “groovy”? Shakespeare used the rich vocabulary of his day within his plays.

fresh in murmur: being rumored
“And then ‘twas fresh in murmur (as you know what great ones do, the less will prattle of that he did seek the love of fair Olivia.” Sea Captain 1.2.29
And then I heard a rumor (because you know how much common people love to gossip about royalty.)

galliard: lively dance in triple time
“What is thy excellence in a galliard, knight?” Sir Toby Belch 1.3.102
How good are you at these fast dances?

gaskins: loose breeches
“That if one break, the other will hold; or if both break, your gaskins fall.” Maria 1.5.21
If one button breaks the other will hold up, but if both break then your pants will fall down.

leman: sweetheart
“‘Twas very good, i’ faith. I sent thee sixpence for the leman; hadst it?” Sir Andrew Aguecheek 2.3.20
I sent you some money to spend on your girlfriend. Did you get it?

baffle: publicly humiliate
“This is open. I will be proud, I will read politic authors, I will baffle Sir Toby, I will wash off gross acquaintance, I will be point-device the very man.” Malvolio 2.5.146
I will be vain, and proud, and I’ll study politics, I’ll insult Sir Toby, and get rid of my lower class friends, and I’ll be the perfect man for her.

aqua vitae: distilled liquors
“Like aqua vitae with a midwife.” Sir Toby Belch 2.5.180
Like medicine for the sick.

conster: explain
“My lady is within, sir. I will conster to them whence you come.” Feste 3.1.50
My lady is inside, tell me where you’re from and I’ll pass it along.

give me leave: do not interrupt me
“Give me leave, beseech you. I did send, After the last enchantment you did here, a ring in chase of you.” Olivia 3.1.102
Let me say something please. After our last enchanted evening I sent a ring after you.

vulgar proof: common knowledge
“No, not a grize; for ’tis a vulgar proof that very oft we pity enemies.” Viola 3.1.115
No, not a bit, it’s commonly known that we feel sorry for our enemies.

license of ink: freedom that writing permits
“Go, write it in a martial hand, be curst and brief.” Sir Toby Belch 3.2.37
Go write it down and make it look like a soldier’s handwriting.

midsummer madness: extreme folly
“Why, this is very midsummer madness.” Olivia 3.4.50
This is insane.