NooB Question; How to counter thornmail as an ADC?
"lifesteal" yadda yadda. Help me out here. :)
"lifesteal" yadda yadda. Help me out here. :)
Life steal and Magic resist counter it.
Thornmail counters lifesteal, so don't build any more lifesteal than you usually would.
Build armor pen. You kill them faster and get more lifesteal out of every hit - meaning that there'll not only be less total instances of recoil to deal with, but also every instance of recoil is better compensated by the lifesteal you already have.
Build armor reduction. but I only recommend it on marksmen that truly benefit from the CDR, like Lucian or Ezreal.
Build MR. Although be aware that you'll feel a hit to your damage output.
If you don't like the life steal answer, play ziggs adc lol
variety of options.
dont forget 
essential.
Lifesteal + Last Whisper are your only REAL options; Magic Resistance and damage reduction help too
Niche ideas:
An assassin meta is literally an ADC player's worst nightmare. I can't tell you how many games I've essentially had 6 defensive items, WHILE AHEAD. Literally a damned nightmare...

wow, incredible reply, and more in depth than i could have hoped for. You've gone beyond the call of duty my friend. honored i thank you so much for your help!! #ADCin2K17 [slayer-pantheon-thumbs]
BT and Scimitar/Bork
one lifesteal item = item countered
two lifesteal items = item made 100% useless
buy a last whisper, dont upgrade then a vamp scepter and rush a life steal item
With current game itemization you can achieve something like 60 lethality with runes. Combine that with a last whisper and that 100 armor melts away.
If you have trouble getting lifesteal because the items are too expensive just play AP Vayne

are your best bets
uhm almost everything counter thornmail
Warlords keystone & stoneborn pact keystone
on your support for example
pick one or 2 of these
for lifesteal as a direct counter
gives damage reduction
simply getting more health will help out too
shreds armor, less armor is less return damage
Amplify your lifesteal
if you are feeling a bit adventurous then just deal some magic damage and steal their mr bypassing thornmails armor and making you more tanky
Combine with runaans to steal up to 75 mr !!
I covered a lot of the options but there are probably more
does countering your opponent sometimes mean altering your preferred build? Yes but in the end it is the stronger option to go for It is ok to build a defensive item because staying alive to deal your dps trumps dying with your maximum cirtbuild everyday
oh and dont forget about runes
if you really feel like dicking enemy ap's which includes thornmail/sunfire and 90% of the tank roster you can get 75 mr from your runepage No mere tank will stop you now